This invention relates to a toner for developing electrostatic images and, more particularly, to a toner suitable for use in a one-component developing system and a two-component developing system. The present invention is also directed to a full color image forming apparatus using the above toner.
In a dry electrophotographic system, a copy is obtained by a method including the steps of forming an electrostatic latent image on a photoconductor, developing the electrostatic latent image with a developer to form a toner image, transferring the toner image onto a copying paper, and heat-fixing the toner image (usually with a heat roller). In the case of a full-color image which requires a high gloss and an excellent color tone, a polyester resin as shown in JP-A-S61-7844 or a polyol resin as shown in JP-A-H07-77832 has been frequently used as a binder resin for the toner.
However, the polyester resin tends to cause formation of toner aggregates in a toner bottle or in a vessel of a developing unit, which cause a void phenomenon in the toner image. Also, when a printed image is contacted with a vinyl chloride mat or the like for a long time, the image tends to adhere to the mat or become sticky. Additionally, when the polyester resin has a high acid value, the resulting toner can obtain a high charge potential but has low environmental fluctuation resistance.
A toner containing the polyol resin has a problem because of low chargeability. In particular, when the toner is used as a single-component developer, the low chargeability results in background stains or a low image density. The problem ascribed to the low chargeability may be overcome, when the toner is used as a two-component developer in conjunction with a carrier. However, when the developer is used for an image forming apparatus employing a small-size developing unit in which the amount of the developer contained in a vessel is small, agitation of the developer cannot sufficiently charge the toner because the frictional forces are not sufficiently generated due to the low weight of the developer. Such insufficient charging becomes serious, especially when the image forming machine is a full color type in which the toner concentration in the color developer is higher as compared with that of a black color developer.